Journal of the Geological Society, London, Vol. 162, 2005, pp. 789–799. Printed in Great Britain.

789

Neoarchaean (c. 2.58 Ga) halite casts: implications for palaeoceanic chemistry

K. A. ERIKSSON 1, E. L. SIMPSON 2, S . MASTER 3 & G. HENRY 4

1Department of Geosciences, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA

(e-mail: kaeson@vt.edu)2Department of Physical Sciences, Kutztown University of Pennsylvania, Kutztown, PA 19530, USA

3EGRI-HAL, School of Geosciences, University of the Witwatersrand, P. Bag 3, WITS 2050, Johannesburg, South Africa4Council for Geoscience, P. Bag X112, Pretoria, 0001, South Africa

Abstract: Possibly the most extensive and best-preserved Archaean halite casts yet discovered occur in the c.

2.58 Ga upper Black Reef and basal Oaktree formations, Transvaal Supergroup, in Mpumalanga Province,

South Africa. Halite casts are isolated on bedding planes, range in size from c. 1 mm to 20 mm, and have

cubic, dumbbell and triangular shapes, as well as hopper-like pyramidal hollows on cube faces. Some of the

casts display distinct hopper shapes characteristic of halite crystals. The halite cast-bearing pavements are

developed within silicified mudstone interbedded with siltstone or stromatolitic dolomite. Associated

sedimentary structures pointing to subaerial exposure include adhesion ripples and warts, desiccation and

prism cracks, rill marks and tepee structures. Halite cast-bearing beds are interpreted as supratidal flat or

sabkha deposits. The presence of isolated casts and hopper-shaped crystals suggests that halite resulted from

displacive growth within the sediment from supersaturated residual brines after mudstone deposition. Absence

of any indication of the former presence of gypsum or anhydrite supports previous contentions that the

Neoarchaean ocean was deficient in sulphate or contained a high bicarbonate to calcium ratio such that with

progressive evaporation, most calcium was consumed before the gypsum stability field was reached. The

association of halite and carbonate in the upper Black Reef and basal Oaktree formations constrains the

palaeolatitude of the Transvaal Basin at 2.58 Ga to subequatorial (10–308).

Keywords: Neoarchaean, seawater, composition, halite, palaeolatitudes.

In modern marginal-marine settings, halite accumulates in

subaerial and subaqueous hypersaline environments including

peritidal flats, and environments not affected by tides or storms

but flooded by marine waters that seep through a physical barrier

separating the evaporitic basin from the ocean (Handford 1991).

In each of these settings, progressive evaporation of seawater

leads to precipitation of calcite and gypsum followed by halite

(Handford 1991). Halite precipitation takes the form of subaqu-

eous cumulates, or subaqueous bottom or intrasediment precipi-

tates (Lowenstein & Hardie 1985). Later influx of fresh or storm

waters results in dissolution of halite such that, in the geological

record, halite is rarely preserved as a mineral but rather as casts

or moulds on bedding planes (Llewellyn 1968; Southgate 1982;

Demicco & Hardie 1994).

In modern settings and in the geological record extending back

to 1.8 Ga, gypsum is a common evaporite mineral often devel-

oped in association with halite. The pre-1.8 Ga record, in

contrast, is almost devoid of gypsum. The lack of gypsum is

considered to have important implications for the composition of

the early ocean (Grotzinger & Kasting 1993).

A number of horizons in the transition beds between the Black

Reef Formation and Oaktree Formation of the Chuniespoort

Group, Transvaal Supergroup, South Africa (Figs 1 and 2)

contain a variety of casts that represent the oldest evidence for

halite precipitation described from the geological record. The

sedimentology of these and associated facies has been investi-

gated along the eastern escarpment in Mpumulanga Province,

South Africa (Fig. 1) with a view to understanding: (1) the

depositional environment of halite precipitation; (2) the mode of

precipitation of halite; (3) the implications for Neoarchaean

ocean chemistry of halite precipitation in rock units that underlie

those containing extensive aragonite crystal pseudomorphs.

Critical to the interpretation of palaeoceanic chemistry is the

unambiguous characterization of former evaporites on the basis

of pseudomorph morphologies.

Geological setting

The Transvaal Supergroup is a late Archaean to early Palaeopro-

terozoic succession of siliciclastic and chemical sedimentary, and

subordinate volcanic rocks that are preserved within three

separate sub-basins: Transvaal, Kanye and Griqualand West

(Catuneanu & Eriksson 1999). Rocks of the Transvaal Super-

group unconformably overlie either the Witwatersrand Super-

group or the Ventersdorp Supergroup and make up the floor

rocks of the Bushveld Complex. In Mpumulanga Province, South

Africa, the Transvaal Supergroup overlies the protobasinal Wolk-

berg Group, and is subdivisible into the Black Reef Formation,

Chuniespoort Group and Pretoria Group (Figs 1 and 2). The

Black Reef Formation is gradational into the Oaktree Formation

at the base of the Chuniespoort Group. A latest Archaean age for

the Black Reef–Oaktree study interval is constrained by U–Pb

dating of tuffs intercalated within the Oaktree Formation NW of

Johannesburg (2550 � 3 Ma and 2588 � 7 Ma) and in Mpumu-

langa Province (2583 � 5Ma) (Walraven & Martini 1995; Martin

et al. 1998).

Along the Mpumulanga escarpment, the Black Reef Formation

varies in thickness from 10 to 20 m (Henry et al. 1990; Eriksson

& Reckzo 1995) and consists of conglomerates, sandstones

and mudstones. A lower upward-fining interval is interpreted

as a braided-alluvial to floodplain transition, whereas two over-

lying upward-coarsening intervals are considered to represent

progradational braid-delta deposits (Fig. 3; Henry et al. 1990).

Braid-delta deposits (sensu McPherson et al. 1987) are confined

to the northern and southern parts of the outcrop belt and the

halite-bearing units are located in the northern portion of the belt

(Fig. 3). The study interval overlies cross-bedded sandstones of

braided-alluvial origin and is capped by large stromatolitic

domes of the Oaktree Formation that are interpreted as subtidal,

marine facies (Truswell & Eriksson 1975; Beukes 1987).

Facies descriptions and interpretations

Sections were measured through halite-bearing intervals in the

upper Black Reef Formation and Oaktree Formation at the base

of the Chuniespoort Group at three locations along the Mpumu-

langa escarpment: Dientje–Old Stone Bridge and Ses-I-se-Draai

(Fig. 4). In addition, a section was measured through the upper

Black Reef Formation and Oaktree Formation at Three Ronda-

wels, where no halite casts were observed.

Pebbly sandstone facies

The lower Black Reef Formation along the escarpment is less

than 5 m thick (Fig. 3; Henry et al. 1990) and consists of pebbly

and coarse-grained sandstones containing medium- to large-scale

trough cross-beds, and small-scale planar cross-beds within

which the foresets are defined by grain flows (see Buck 1985).

Palaeocurrent data indicate strongly unidirectional flow towards

the west and SW (Fig. 5). Pebbly sandstones of the lower Black

Fig. 1. Geological map of the Transvaal

Supergroup along the eastern Mpumulanga

escarpment with inset of South Africa. Also

shown are the locations of the measured

sections (Fig. 4).

K. A. ERIKSSON ET AL .790

Reef Formation are sharply overlain by siltstone–shale, stromato-

litic dolomite or tuff (Fig. 4). Large-scale, symmetrical ripples

with wavelengths up to 20 cm (Fig. 6a) define the top of the

pebbly sandstone facies at Ses-I-se-Draai (Fig. 4b).

The lower Black Reef Formation has been interpreted as a

mainly braided-alluvial deposit by Henry et al. (1990) on the

basis of its coarse grain size and dominance of unidirectional

cross-beds. Comparable facies are developed in modern (e.g.

Coleman 1969; Cant & Walker 1978), and ancient braided-

alluvial deposits including those of Archaean age (e.g. Eriksson

1978; Beukes & Cairncross 1991; Els 1998). The lack of

meandering-river deposits in the Black Reef Formation is

consistent with an absence of bank stabilization by vegetation in

the Neoarchaean landscape (see Schumm 1968). Large symme-

trical ripples developed on the top of the lower Black Reef

Formation reflect wave reworking associated with initial trans-

gression. Similar ripples have been described from relict Pleisto-

cene sediments that were reworked during the Holocene sea-level

rise (Leckie 1988).

Fine- to medium-grained sandstone facies

This facies dominates the Three Rondawels section, where it is

over 5 m thick (Fig. 4c). Sandstone is fine- to medium-grained

and has horizontal stratification and small-scale trough cross-

beds. Bedding planes exhibit a range of sedimentary structures

including adhesion warts and ripples (Fig. 6b), desiccation cracks

that display evidence for multiple generations of shrinkage and

infill (Fig. 6c), raindrop impressions preserved as casts with

crater rims (Fig. 6d), aligned, lenticular sandstone dykelets

developed on desiccated mudstone polygons (Fig. 6e), and

various ripples including symmetrical, asymmetrical, ladderback

and interference forms; some ripples contain desiccated mud-

stone drapes.

The association of sedimentary structures in this facies

indicates very shallow water and periodically emergent condi-

tions. Adhesion structures are produced by wind-blown sand

adhering to a wet or damp surface (Kocurek & Fielder 1982).

Such conditions would also promote the formation and preserva-

tion of raindrop impressions. Alternating submergence and

emergence of the depositional surface is indicated by the

desiccation cracks. The symmetrical and interference ripple

Fig. 2. Stratigraphic column for Transvaal Supergroup (modified from

Catuneanu & Eriksson 1999).

Fig. 3. Cross-section illustrating the

geometry and facies of the Black Reef

Formation along the Mpumulanga

escarpment (modified from Henry et al.

1990). c.u., coarsening upwards; f.u., fining

upwards.

NEOARCHAEAN HALITE CASTS AND OCEAN CHEMISTRY 791

forms indicate an environment influenced by waves whereas the

ladderback forms support an intertidal setting (see Klein 1985).

Suspension settling of mud on ripples during slack-water periods

was followed by exposure and desiccation. The origin of the

lenticular sandstone dykelets is more problematic. Geometrically

they resemble syneraesis cracks (e.g. van Straaten 1954) but such

an origin would not explain their preferred alignment. A

plausible explanation is that a mud layer cracked subaqueously

in response to downslope creep of a semi-consolidated mud layer

with the cracks subsequently infilled with sand. A similar origin

has been inferred for linear shrinkage cracks in the Green River

Formation (Picard 1966).

Siltstone–mudstone facies

Massive and laminated siltstone and mudstone form the upper

Black Reef Formation at Ses-I-se-Draai (Fig. 4b). Traction-

produced sedimentary structures are notably lacking. This facies

is also interbedded with stromatolitic dolomite of the lower

Oaktree Formation at the same location. Sedimentary structures

are dominated by horizontal laminations, but symmetrical ripples

as well as starved ripples of siltstone within mudstone are present

locally. Symmetrical ripple crests vary in orientation from

north–south to east–west.

Lack of traction-produced structures in this facies in the Black

Reef Formation indicates slow suspension sedimentation below

wave base. The presence of ripples reflects gradual shoaling into

the basal Oaktree Formation.

Silicified mudstone–siltstone facies

This facies is developed in the lower Oaktree Formation at Ses-I-

se-Draai and Old Stone Bridge (Fig. 4a and b) and as eight

horizons interbedded with sandstone in the upper Black Reef

Formation at God’s Window (Fig. 1). Individual horizons range

in thickness from 2 to 30 cm. Facies are dominated by silicified

Fig. 4. Detailed measured sections through

the Black Reef Formation and basal

Oaktree Formation. (a) Old Stone Bridge–

Dientjie; (b) Ses-I-se-Draai; (c) Three

Rondawels. (See Figure 1 for locations of

cross-sections.)

K. A. ERIKSSON ET AL .792

mudstone in laminae between 2 and 30 mm thick. Mudstone is

mainly massive with rare faint parallel laminations. Intercalated

within the mudstone are 1 to 4 mm thick, massive, graded-

bedded and rippled, lenticular-bedded siltstone laminae (Figs 6f

and 7a) that locally infill desiccation cracks and angular depres-

sions in the underlying mudstone (Fig. 7b). Locally, scouring is

present at the base of siltstone beds. Chaotic intraclast breccias

are common within this facies typically in association with

small-scale syndepositional faults and dismembered folds over-

lain by intact laminations. Other structures developed in this

facies include halite casts in extensive pavements up to 5 m by

20 m, rare tepee structures (Fig. 7c), rill marks, and desiccation

and prism cracks (Fig. 7a). Halite casts range from ,1 cm to

2 cm in size, vary from square- to triangular- to dumbbell- to

hopper-shaped, and typically are isolated from one another (Figs

7d and 8a–c). Casts commonly display internal zoning (Fig. 8d)

and hopper-like pyramidal hollows on cube faces, and are

commonly associated with desiccation cracks.

Evidence for the former presence of sulphates is lacking.

Specifically, the following criteria are not evident on outcrop or

in thin section: (1) nodules of former anhydrite replaced by

quartz, calcite or dolomite; (2) pseudomorphs after swallow-tail

gypsum or after anhydrite laths; (3) displacive crystallization

associated with non-cubic pseudomorphs (see Demicco & Hardie

1994). Radiating crystal fans in overlying limestones that were

previously interpreted as gypsum pseudomorphs (Bertrand-Sarfa-

ti 1976) are now considered to be the product of neomorphism

of botryoids of aragonite, on the basis of textures and elevated

strontium contents (Sumner & Grotzinger 2000).

Associations of lithologies in this facies indicate overall low-

energy conditions, favouring mud accumulation, interrupted by

frequent higher-energy pulses that introduced silt to the deposi-

tional setting. Massive siltstone laminae probably reflect storm

processes whereas ripple cross-laminae record weak traction

reworking. Desiccation structures indicate periodic exposure of

the depositional surface. The rare tepee structures may have

formed as a result of the expansive growth of halite and/or early

carbonate cement in the zone of evaporative pumping (see

Warren 1983; Lowenstein & Hardie 1985). Chaotic breccias are

mainly products of desiccation and/or incipient tepee formation.

Isolated halite casts and the presence of hoppers with concave

margins support displacive growth of halite probably as a result

of evaporation of capillary brines (see Shearman 1978; Gornitz

& Schreiber 1981; Handford 1988). Less common casts with

internal zoning support incorporative growth within the sediment

(see Handford 1988). Halite formation by upward rather than

downward diffusion is favoured by the shallow-water setting

implied by the desiccation cracks (Handford 1988). Angular

depressions at the base of siltstone laminae represent casts of

halite that was dissolved by lower-salinity waters that introduced

silt. Casts indicate that halite precipitation occurred at very

shallow depths within the sediment or on the sediment surface.

Stromatolitic dolomite facies

Stromatolitic dolomite is the predominant facies in the Oaktree

Formation and, in general, the size of stromatolitic domes

increases upwards (Fig. 4). At Ses-I-se-Draai, silicified stromato-

lite bearing horizons between 5 and 25 cm thick are interbedded

with siltstone–mudstone, tuff and carbonate mudstone–grain-

stone (Fig. 4b). Stromatolites in these horizons consist of linked,

low-relief domes, or vertically stacked domes that increase in

width and complexity upwards (Fig. 8d). Laminations on a

millimetre scale are well preserved by secondary silicification.

Stromatolitic domes higher in the Oaktree Formation are up to

5 m wide, display relief of up to 50 cm, and also are character-

ized by well-defined millimetre-scale laminations. Secondary

silicification is lacking. Stromatolites are elongated east–west to

SE–NW.

All structures documented in this facies are similar to

isopachously laminated stromatolites that consist of encrusting

layers of former (high-magnesium?) calcite, and have been

described from stratigraphic units higher up in the Chuniespoort

Group and from other Archaean and Palaeoproterozoic succes-

sions (Grotzinger & James 2000; Pope & Grotzinger 2000). The

encrusting layers are interpreted as products of abiotic precipita-

tion of carbonate mud related to progressive oversaturation of

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Fig. 5. Palaeocurrent data for braided-alluvial facies of the lower Black

Reef Formation.

NEOARCHAEAN HALITE CASTS AND OCEAN CHEMISTRY 793

K. A. ERIKSSON ET AL .794

seawater as a result of increasing temperature and salinity (Pope

& Grotzinger 2000). Radiating crystal pseudomorphs indicative

of aragonite precipitation (Sumner & Grotzinger 2000) were not

identified but these are commonly destroyed by dolomitization

and silicification (C. Schreiber, pers. comm.). Thus, the former

presence of aragonite cannot be excluded. The upward increase

in size of domical stromatolites (Fig. 4) is considered to reflect

progressive deepening.

Fig. 6. Photographs of facies. (a) Large-scale symmetrical ripples defining a ravinement zone above the lower Black Reef Formation at Ses-I-se-Draai. (b)

Adhesion ripples or warts in fine- to medium-grained sandstone facies of the upper Black Reef Formation at Three Rondawels. (c) Desiccation cracks

displaying multiple generations of shrinkage and infill in fine- to medium-grained sandstone facies at Three Rondawels (scale is 6.5 cm long). (d)

Raindrop casts in fine-grained sandstone at Three Rondawels. (e) Oriented, lenticular sandstone dykelets in fine- to medium-grained sandstone facies at

Three Rondawels. (f) Photomicrograph showing graded siltstone laminae within silicified mudstone at Ses-I-se-Draai. Angular depressions at the base of

some siltstone laminae represent infilled halite casts. Photograph is 2.5 cm wide.

Fig. 7. Photographs of facies. (a) Photomicrograph showing graded-bedded and rippled lenticular-bedded siltstone laminae, and prism cracks within

silicified mudstone at Ses-I-se-Draai. Photograph is 2.5 cm wide. (b) Photomicrograph showing a halite mould infilled with siltstone. Photograph is 1.5 cm

wide. (c) Tepee structure in silicified siltstone–sandstone facies at Ses-I-se-Draai (scale is 15 cm long). (d) Casts of halite hopper crystals from silicified

siltstone–mudstone facies at Ses-I-se-Draai. Some casts are internally zoned and indicate incorporation of sediment during growth. Casts with concave

margins provide evidence for displacive growth of halite (scale is in centimetres).

NEOARCHAEAN HALITE CASTS AND OCEAN CHEMISTRY 795

Mafic tuff

Mafic tuffs are developed in each of the measured sections and

range in thickness from 50 cm to 5 m (Fig. 4). Locally, the tuffs

contain glass shards up to 1 cm in length. The tuffs are mostly

massive but locally display evidence of reworking in the form of

horizontal stratification and symmetrical ripples. Halite casts

similar in size and structure to those discussed above are devel-

oped on the top of the tuff bed at Ses-I-se-Draai.

Tuffaceous horizons are widespread in the lower part of the

Chuniespoort Group (Walraven & Martini 1995; Martin et al.

1998) but the location(s) of the explosive volcanic centres is not

known. Also unclear is whether the tuffaceous horizons in

different parts of the basin reflect a single or multiple explosive

events. In the study area, deposition of the tuffs occurred in

shallow water, as implied by the traction-produced structures. In

addition, the local presence of halite casts supports a shallow-

water, evaporitic setting.

Palaeolatitudinal constraints

Because certain rock types such as evaporites (halite, gypsum

and anhydrite), carbonates, coals and tillites are climatically

sensitive sediments, and tend to be deposited under restricted

conditions, they are useful in palaeoclimatic studies. Evaporites

occur in the subtropics, where it is dry, and where evaporation

exceeds the total of precipitation plus inflow of surface water

(Gordon 1975). Carbonates, in particular those of the Bahamian

type, occur in equatorial, subtropical, and warm temperate

regions, where it is warm and where there is adequate sunlight

penetration (Scotese & Barrett 1990).

Fig. 8. Photographs of halite casts. (a) Casts of halite hopper crystals in silicified shale, Lower Oaktree Formation, near Bourke’s Luck potholes. The cast

in the centre shows internal zoning as a result of sediment incorporation during growth (scale in centimetres). (b) Halite casts in silicified mudstone,

Lower Oaktree Formation, near Burke’s Luck potholes. The large halite cast, 15 mm across, and triangular or tricuspate shapes of casts of the corners of

halite crystals, should be noted. (c) Casts of hopper-shaped halite cubes with depressed cubic faces, as evidence for displacive growth, from silicified

siltstone–mudstone facies at Ses-I-se-Draai (coin is 1 cm in diameter). (d) Small stromatolite domes displaying an upward increase in width in

stromatolitic dolomite facies at Ses-I-se-Draai (coin is 2 cm in diameter).

K. A. ERIKSSON ET AL .796

The distribution of climatically sensitive sedimentary rocks

has been used to independently test and verify the palaeolatitudes

calculated from palaeomagnetic studies assuming a geocentric

axial dipole model for the Earth’s magnetic field in the past

(Irving & Briden 1962; Opdyke 1962; Briden 1968, 1970). For

the Mesozoic and Cenozoic, palaeolatitudinal positions of the

major continents are known with great precision through the use

of seafloor magnetic anomalies (Ziegler et al. 1983). Detailed

palaeomagnetic studies have also allowed for accurate recon-

structions of Palaeozoic palaeogeography (Cocks & Torsvik

2002; Torsvik & Cocks 2004), but palaeomagnetic reconstruc-

tions for the Precambrian are much less certain, because of the

paucity of well-defined ages. For the Mesozoic and Cenozoic,

Scotese & Barrett (1990) plotted the latitudinal distribution of

known climatically sensitive sedimentary rocks (evaporites,

carbonates, coals and tillites) in the form of pole-to-pole

histograms. They successfully used these histograms and their

associated probability functions to calculate a Palaeozoic Appar-

ent Polar Wander (APW) path for the Gondwana Supercontinent,

which is in fairly good agreement with the palaeomagnetically

determined APW of Bachtadze & Briden (1990). Scotese &

Barrett (1990) showed that climatically sensitive sedimentary

rocks can be used to successfully predict the location of

palaeolatitudes, and hence of palaeopoles, assuming that the

zonal distribution of climate patterns was the same throughout

the Phanerozoic eon as it is today. This assumption can be used

for most of Precambrian Earth history, except for those periods

when the equator-to-pole temperature gradient was very different

from now, such as during the global periods of glaciation during

the Palaeoproterozoic and Neoproterozoic eras.

The analytical results of Scotese & Barrett (1990), based

partly on the data of Parrish et al. (1982), show that evaporites

are restricted mainly to latitudes (N or S) of between 58 and 358,

with an occurrence probability of 0.72, and most modern

carbonates are restricted to latitudes (N or S) of between 108 and

308, with an occurrence probability of 0.55. Thus, for the

Transvaal Supergroup halite–carbonate association, there is a

maximum probability that this pair of climatically sensitive

lithologies was formed at between 108 and 308 palaeolatitude at

c. 2.58 Ga. This subequatorial palaeolatitude result fills an

important gap in our palaeogeographical knowledge of the

Transvaal Supergroup, because the only well-constrained palaeo-

magnetic palaeopole from this sequence is from the much

younger 2222 � 13 Ma Ongeluk lavas (Cornell et al. 1996).

Evans et al. (1997) inferred an equatorial palaeolatitude

(118 � 58) for the Ongeluk lavas; these occur in the Postmasburg

Group that overlies the Ghaap Group, a correlative of the

Chuniespoort Group. The preserved rocks of the Transvaal

Supergroup thus appear to have been deposited while the

Kaapvaal Craton was situated in low palaeolatitudes, ,308 N or

S. Because of the absence of palaeomagnetic data, little can be

said about the movement of the Kaapvaal Craton between the

period of deposition of the Black Reef Quartzite Formation and

outpouring of the Ongeluk lavas. However, a 64.5 � 17.58

palaeolatitude for the 2782 � 5 Ma Derdepoort basalt (Wingate

1998) suggests a northward migration of the Kaapvaal Craton

between 2.8 and 2.2 Ga.

Discussion

Vertical successions of facies (Fig. 4) suggest progressive

deepening from alluvial settings in the lower Black Reef

Formation to a subtidal environment at the time of deposition of

the Oaktree Formation. The presence of stromatolites produced

by precipitation from oversaturated seawater supports a marine

rather than a lacustrine depositional environment. In the Ses-I-se-

Draai location, wave ripples developed above alluvial facies of

the lower Black Reef Formation define a ravinement surface and,

together with the overlying facies, indicate rapid deepening. The

upper Black Reef Formation at this locality consists of a

progradational parasequence that was followed by deepening into

the basal Oaktree Formation. A similar vertical transition (except

for the later deepening phase) from continental sandstone to thin

transgressive-phase deposits followed by thicker subtidal to

supratidal, regressive-phase deposits is well documented from

sabkhas of Abu Dhabi (Kinsman & Park 1976; Wright 1984).

The parasequence at Ses-I-se-Draai provides constraints on the

depositional setting of the halite-cast hosting facies and by

implication the environment of halite precipitation. Lack of

traction-produced structures in the siltstone and mudstone facies

at the base of the parasequence implies a sub-wave base

environment, whereas evidence for exposure throughout the

upper half of the parasequence supports a peritidal setting in

which small-scale stromatolitic domes and carbonate muds and

sands together with evaporites were deposited. The preponder-

ance of evidence for intrasediment growth and dissolution of

evaporites supports a sabkha and, in particular, a saline mudflat

setting (see Handford 1991; Demicco & Hardie 1994). In such

an environment, evaporite crystals close to the surface commonly

dissolve during floods, resulting in crystal moulds, although a

decrease in salinity could also be related to influx of marine

waters during storms.

The geomorphology of the sabkha setting can be evaluated

with reference to the cross-section of the Black Reef Formation

(Fig. 3). This section utilized the top of the last coarse-grained

facies as the datum. If the cross-section is hung from the base of

the first occurrence of large-scale, stromatolitic domes, it would

demonstrate that the halite-bearing and associated facies occupy

the northern margin of the fan-delta deposits. In a palaeogeogra-

phical sense, this depression may have taken the form of an

embayment between a fan delta to the south and an undocumen-

ted fan delta further north. If so, it is likely that halite-bearing

facies may also be present between the two fan deltas shown on

the cross-section (Fig. 3), but poor outcrop precludes testing of

this model.

The lack of evidence for sulphate minerals in the studied

sections has important implications for Neoarchaean palaeocea-

nic and/or palaeoatmospheric chemistry. Rare gypsum pseudo-

morphs reported from the Neoarchaean Carawine Dolomite in

the Hamersley Basin, Australia (Simonson et al. 1993) represent

the oldest evidence of gypsum precipitation. Mesoarchaean barite

from the Warrawoona Group in the Pilbara Block of Australia,

previously interpreted as a replacement of gypsum (e.g. Buick &

Dunlop 1990), is now considered to represent a primary hydro-

thermal precipitate (Runnegar et al. 2001). Evaporite pseudo-

morphs from the Onverwacht Group in the Barberton Greenstone

Belt, South Africa, represent silicified nahcolite, a sodium

bicarbonate (Lowe & Worrell 1999). The virtual absence of

gypsum from the early Earth record is attributed to low sulphate

concentrations in early Precambrian oceans related to the anoxic

state of the atmosphere or to a high bicarbonate-to-carbonate

ratio in early Precambrian oceans such that during progressive

evaporation calcium would have been exhausted before the

gypsum field was reached (Grotzinger & Kasting 1993). The

results of this study support the model of Grotzinger & Kasting

(1993) but do not resolve the alternative interpretations for the

absence of gypsum. The presence of aragonite crystal pseudo-

morphs that make up as much as 50% of Neoarchaean carbonate

NEOARCHAEAN HALITE CASTS AND OCEAN CHEMISTRY 797

successions including the Malmani Dolomite indicates over-

saturation of the sea-water with respect to calcium carbonate

(Grotzinger & James 2000; Sumner & Grotzinger 2000). How-

ever, the lack of any ferric iron pigmentation in the Black Reef

alluvial facies (Twist & Cheney 1986) and in older Archaean

alluvial facies such as the Moodies Group and Pongola and

Witwatersrand Supergroups (Eriksson 1978; Beukes & Cairn-

cross 1991; Els 1998) indicates overall anoxic atmospheric

conditions favouring a low sulphate content of the oceans.

Conclusions

(1) Halite cast-bearing beds in the 2.58 Ga upper Black Reef and

lower Oaktree formations accumulated in supratidal or sabkha

palaeoenvironments.

(2) Halite precipitation occurred by displacive growth within

the host sediment.

(3) Evidence for the former presence of sulphates is lacking,

thereby supporting previous hypotheses that the Neoarchaean

ocean was deficient in sulphate or contained anomalously high

bicarbonate contents.

(4) A subequatorial palaeolatitude for the Transvaal Basin at

2.58 Ga is implied by the association of carbonate and halite,

and provides a new data point for palaeogeographical reconstruc-

tions.

Field work on which this paper is based was funded by National

Geographic Society grant 6003-97. We thank C. Schreiber and P. Turner

for their insightful reviews, and M. Fowler for his constructive com-

ments.

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Received 15 September 2004; revised typescript accepted 13 January 2005.

Scientific editing by Mike Fowler

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